When I was reaching Reception, I taught one child who regularly fell asleep during singing and afternoon assemblies. The first thing I did was to speak to the parents to check whether he still slept in the afternoons at weekends.
When it turned out that he regularly still had a sleep, I suggested to his mum that he would benefit from doing more half days at school, or shorter days until we both felt that he was ready for full time school again.
Alternatively, it could be that your ds just finds the music very relaxing!